The present invention relates to the processing of clay minerals and, more particularly, is directed to the processing of kaolinitic clays at high solids under acidic conditions to produce an improved paper filler composition.
It is well known that the incorporation of clay-based filler compositions into the paper web during the formation of paper sheet can significantly improve the optical properties of the resultant paper sheet. This improvement in opacity of the paper sheet by means of incorporation of the clay filler is the result of increased light scattering due primarily to the difference in indices of refraction between the filler, the paper fiber and air, and also due to the increased number of light scattering voids formed in the paper web upon the incorporation of a clay filler. In order for the clay filler to perform well in improving opacity of the paper, the particle size distribution within the clay filler must be such that a large percentage of the clay particles have equivalent spherical diameters of between 0.6 and 1.5 microns and contain minimal particles with equivalent spherical diameters larger than 45 microns. Further, the paper maker must provide a paper product which meets customer specifications as to color shade. As it is customary to use dyes to obtain the desired paper color, the clay filler should have a consistent color, preferably showing good whiteness and little red, green, yellow or blue shade.
Unfortunately, very few kaolinitic clays in their crude state have the particle size distribution or the color characteristics which are required for a good paper filler composition. For example, although tertiary Kaolin clays mined in East Georgia have a fairly good particle size distribution in the crude state, they are usually highly colored. Additionally, their coloration varies widely ranging from grey-white to cream to brown to yellow to pink to purple. Accordingly, such East Georgia tertiary Kaolins are not, at present, widely used to make paper filler for white paper production.
Therefore, it has become customary in the industry to beneficiate the crude kaolinitic clays used in clay filler compositions for paper making to improve particle size distribution and also improve color characteristics by removing ferric iron-containing compounds in the clay. Such ferric iron-containing compounds impart a non-white color to the clay and reduce the overall brightness or reflectance to visible light of the clay. It is well known that the effect of these ferric iron-containing compounds may be reduced by treating the clay with a reducing agent which converts the ferric ion to the less highly colored ferrous ion. A variety of reducing agents are known to be suitable for treating kaolinitic clays, but the most commonly used reducing agents are water-soluble dithionites or sulphites, such as sodium dithionite, zinc dithionite, sodium.bisulphite, sodium hydrosulphite, and sodium pyrosulphite.
In the conventional process for reducing the ferric iron-containing impurities in a kaolinitic clay to the ferrous state, a low solids aqueous suspension of the crude clay is first formed, then if desired, degritted to remove large particles, and then treated with a reducing agent to convert the ferric ions therein to the ferrous state. The ferrous ion is generally very soluble in water and will pass into the water in which the clay is suspended. The treated clay is then thickened, dewatered by filtration and the resultant filter cake thermally dried to produce a clay filler product having a high solids content, at least about 65%, suitable for economic transport. Such a low solid content process requires that the clay suspension be in a fluid state, that is, that the solids content of the crude clay suspension be less than about 50% by weight and usually in the range of 20% to 35% by weight. Unfortunately, such low solids processing of the crude kaolin requires that significant dewatering and drying be required to ready the treated clay product for economic transport. Significant economic benefits would be obtained if the crude kaolin clay could be processed at a high solids content, that is, at least about 65% by weight, so that the dewatering and subsequent drying of the treated clay could be minimized if not eliminated.
One such process for treating kaolinitic clays at high solids is disclosed in U.S. Pat. No. 4,186,027. As disclosed therein, a suspension of raw kaolinitic clay is formed in water at a solids content of 60% to 75% and at a pH in the range of from 7.0 to 11.0 with a dispersing agent. The fluid suspension of clay containing the dispersing agent is then treated with a water-soluble bleaching agent under alkaline pH conditions for a time sufficient to give the desired improvement and brightness of the clay. The treated clay product is said to not require any. dewatering prior to shipment. This patent teaches that the suspensions must be prepared at a pH in the range of 7 to 11 in that suspensions outside of that range are allegedly too viscous to be refined successfully in a scroll-type centrifuge to give a product which is substantially free of particles having an equivalent spherical diameter larger than 10 microns. A drawback of such a high solids processing of the kaolinitic clay at an alkaline pH is that most of the papermaking processess used in the United States are carried out under acidic conditions. Therefore, it would be necessary to add additional acidic compounds in the papermaking process to neutralize an alkaline clay filler material produced from clay processing at a alkaline pH as disclosed in U.S. Pat. No. 4,186,027. Additionally, the clay filler material produced at an alkaline pH does not have good viscosity stability in that the viscosity of the clay filler produced in accordance with that process shows a significant increase in viscosity over time in storage. Further, beneficiation at an alkaline pH does not result in significant color improvement over the color characteristics of the crude.
In the present invention, it has been discovered that contrary to the teachings of U.S. Pat. No. 4,186,027, kaolinitic clays can be beneficiated at high solids, i.e. in excess of at least 65% by weight, under an acidic pH in the range of 5.5 to 7.0 to produce a paper filler product having improved brightness and color characteristics.